1. Field of the Invention
The present invention relates to a thermal head, and more particularly, relates to a thermal head for executing thermal recording on a thermal recording material.
2. Explanation of the Prior Art
In a color thermal printer, full color image is recorded on a color thermal recording sheet during transportation thereof by executing thermal recording thereon. The color thermal recording includes a cyan thermal coloring layer, a magenta thermal coloring layer, and a yellow thermal coloring layer, overlaid on a support in sequence on one another. In addition, a transparent protecting layer is provided on the yellow thermal coloring layer. Thermal recording of the image is executed on yellow, magenta and cyan thermal coloring layers in sequence. After executing thermal recording on the yellow and magenta thermal coloring layers, near ultraviolet rays of which the peak value is 420 nm, and ultraviolet rays of which the peak value is 365 nm, are respectively irradiated, to optically fix the yellow and magenta images. The protective layer protects the thermal coloring layers and imparts glossiness thereon, thereby increasing printing quality.
The thermal head includes a vitreous glaze for storing heat on which a heat-generating resistor, individual (discrete) and common electrodes are overlaid. The protective layer covers the surface of the thermal head. JP-A 10-24615 discloses a thermal head, in which a heat reserving layer is formed on a thermal dissipating substrate by evaporation and a thermal diffusion control layer is provided between an electrically insulating substrate and the heat reserving layer in order to prevent degradation of heat insulating ability of the insulating substrate. Moreover, JP-B No. 3057813 discloses a thermal head which includes a beltlike thermal insulating layer provided inside a glaze layer formed on the surface of the insulating substrate along a main scan direction, in order to utilize heat generated at the heat-generating resistor.
There is a printer in which a color thermal recording sheet is pressed during thermal recording by a thermal head of a partial glaze type provided with a cylindrical partial glaze layer on the insulating substrate. In the thermal head of the partial glaze type, heat generated at the heat-generating resistor is discharged through the distal end of the insulating substrate downstream in a sheet supply direction and through the thermal dissipating layer adhered to the insulating substrate. According to heat dissipation, temperature of downstream side of the protective layer in the sheet transporting direction becomes low compared to that near the heat-generating resistors.
When executing thermal recording on the recording sheet, particularly for a cyan image which has lowest heat sensitivity among three thermal coloring images, steam occurs on recording sheet due to increased temperature thereof. The steam or vapor condensates on the surface of the protecting layer downstream in a supply direction where temperature is low, so that water drops are generated. However, it is likely to occur printing failure such as recession on recording sheet, since water drop on the protecting layer is stacked to recording sheet.
In order to solve the above problems, a conventional thermal head includes a heater attached to lower portion of the thermal dissipating substrate or a heat sink. The thermal head is entirely heated by driving the heater during thermal recording, whereby temperature of the protecting layer downstream in a supply direction increases. This prevents condensation of vapor.
However, the thermal head with a heater is costly. Moreover, power consumption for the thermal head entirely increases because driving power for the heater is required. Furthermore, the thermal head is not cooled enough due to entire heating of the thermal head for the purpose of preventing condensation. Therefore, the thermal head is likely to be overheated, so it is difficult to implement continuous printing.